The decompression chamber (pressure chamber) was a standard-equipment chamber located in the medical lab on board Constitution-class starships. It was designed to treat illnesses caused by decompression sickness, such as that suffered by crewmen exposed to explosive decompression and the vacuum of space. The atmospheric pressure in the chamber was measured by inches of mercury.
The decompression chamber aboard the USS Enterprise saw other uses as well. In 2267, Khan Noonien Singh used the Enterprise's decompression chamber to torture Captain James T. Kirk and attempt to induce the rest of the senior staff to join his cause. (TOS: "Space Seed")